Thread wound golf ball

Abstract: Disclosed is a thread wound golf ball having uniform flight performances, which causes little scattering of weight and inclusion of air into the liquid center, such that no separation of the paste occurs. The thread wound golf ball comprises a liquid center, a thread rubber layer and a cover. The liquid center comprises a paste and a center bag for coating the paste. The viscosity at 23°C (measured by a B type viscometer) of the paste is in the range of from 15 to 70 poise.

Description: The present invention relates to a thread wound golf ball. More particularly, it relates to a thread wound golf ball having a paste-type liquid center. Thread wound golf balls are classified roughly into two types; golf balls having a solid center (center made of a rubber composition) and golf balls having a liquid center. Regarding former thread wound golf balls having a solid center, the amount of spin produced is too large, so that the flight distance becomes short, thereby imparting hard feeling (shot feel ). Therefore, professional golfers and advanced players do not like golf balls with solid centers. The latter liquid center is classified roughly into two types; a liquid center containing paste in which a filler is formulated to adjust the specific gravity and a liquid center containing a liquid which is similar to water. When using the liquid which is similar to water, a rubber center bag for covering (containing) the liquid must have a certain degree of a specific gravity so that the hardness of the center bag becomes high, thereby imparting hard feeling in comparison with the liquid center containing paste. Furthermore, the amount of spin becomes too large so that the flight distance becomes short. To the contrary, regarding the liquid center containing paste in which a filler is formulated to adjust the specific gravity, the paste itself has a certain degree of specific gravity so that the specific gravity of the center bag can be decreased and the hardness can also be decreased, thereby softening the center of the ball. Accordingly, the shot feel becomes soft and the amount of spin does not become too large, which results in good flight performances. Therefore, professional golfers and advanced players like golf balls with a paste type liquid center. However, regarding the liquid center containing paste, scattering of quality (e.g scattering of weight, inclusion of air, deviation of center of gravity in the liquid center due to separation of paste into water and filler, etc.) is liable to arise, which influences the flight performances of the ball. The resulting flight performances are also liable to be scattered. FR-A-816,243 relates to a thread wound golf ball having a liquid center comprising a paste. It has been found that, by using a paste having a viscosity in the range of from 15 to 70 poise at 23°C (measured by a B type viscometer), a liquid center which causes little scattering of weight and inclusion of air into the liquid center, wherein no separation of the paste arises can be prepared. The present invention provides a thread wound golf ball comprising a liquid center, a thread rubber layer winding and a cover, wherein the liquid center comprises a paste and a center bag for coating the paste wherein the viscosity at 23°C (measured by a B type viscometer) of the paste is in the range of from 15 to 70 poise. The present invention also provides a method of making a thread wound golf ball as recited in claim 7. The main object of the present invention is to provide a thread wound golf ball having uniform flight performances, wherein the above problems (e.g. scattering of weight and quality of the liquid center, separation of paste, etc.) are solved. This object as well as other objects and advantages of the present invention will become apparent to those skilled in the art from the following description with reference to the accompanying drawing of which: Fig. 1 is a schematic cross section illustrating one embodiment of the thread wound golf ball of the present invention. The paste may be prepared by formulating freezing-point depressants such as glycerine, fillers for adjusting specific gravity, viscosity modifiers, etc. in water. As the filler for adjusting specific gravity, there can be normally used barium sulphate (BaSO 4 ), sodium sulphate (Na 2 SO 4 ) or mixtures thereof and the like, but is not limited thereto. As the viscosity modifier, there can be used bentonite clay, natural hectorite or mixtures thereof and the like, but is not limited thereto. In the present invention, the viscosity at 23°C (measured by a B type viscometer) of the paste to be used is specified within a range of from 15 to 70 poise. The reason is as follows: When the viscosity of the paste (i.e. viscosity at 23°C measured by a B type viscometer of the paste, which is merely referred to as "viscosity", hereinafter) is smaller than 15 poise, the paste is liable to separate into water and a filler when standing after preparation, thereby causing deviation of gravity of the center. Therefore, the flight performances are scattered. Further, the paste is pelletized by injecting into a mould during the preparation of the liquid center. In this case, the paste is liable to escape from the mould such that it becomes difficult to prepare a spherical pellet. When the viscosity of the paste is larger than 70 poise, the paste becomes too hard and, therefore, entrapment of air is liable to arise. Therefore, scattering of weight is liable to arise and, further, fluidity of the paste becomes inferior so that it becomes difficult to prepare a spherical pellet. Accordingly, it is desired that the viscosity of the paste is set within a narrow range included in the above range. It is particularly preferred that the viscosity is within a range of from 25 to 60 poise. The thread wound golf ball of the present invention is prepared by covering the paste with a center bag to form a liquid center, winding a thread rubber around the liquid center to form a thread rubber layer and then covering the thread rubber layer with a cover. Hereinafter, a structure of the thread wound golf ball of the present invention will be explained with reference to the accompanying drawing. Fig. 1 is a schematic cross section illustrating one embodiment of the thread wound golf ball of the present invention. In Fig. 1, 1 is a liquid center which is prepared by covering a paste la with a center bag lb. 2 is a thread rubber layer which is formed by winding a thread rubber around the liquid center 1. 3 is a cover which is formed by covering a cover on a so-called thread wound core comprising the liquid center 1 and the thread rubber layer 2. Further, a plurality of dimples 3a, preferably 350 to 450 dimples, more preferably 420 ± 25 dimples are provided on the cover 3. It is preferred that the total volume of the dimples 3a is 280 to 340 mm 3 , particularly 300 to 335 mm 3 . The preparation method of the liquid center 1 will be briefly explained below. Firstly, a paste la is injected into a mould and frozen to form a spherical pellet, or injected in a half of the mould and frozen and then the two halves of the mould are combined with each other to form a spherical pellet. A center bag 1b is composed of a vulcanized rubber composition which has a sheet-like form before vulcanization. The center bag is covered on the spherical pellet of the paste, which is then vulcanized. As a result, a liquid center 1 as shown in Fig. 1 can be obtained. When a non-vulcanized sheet of the center bag is covered on the spherical pellet of the paste, for example, the non-vulcanized sheet of a rubber composition is placed on the inner surface of one half of the mould and the spherical pellet of the paste is placed thereon, and then the pellet is covered with the non-vulcanized sheet of the rubber composition. Thereafter, the other half of the mould is put thereon and the rubber composition is vulcanized to prepare a liquid center 1 of a spherical paste la and a center bag 1b. Further, there can also be used a method of combining two halves of the pre-vulcanized center bag in a paste solution to prepare a liquid center, and the preparation method of the liquid center is not specifically limited. The thread rubber layer 2 is formed by winding a thread rubber around the liquid center 1 of which hardness and shape retention are imparted by freezing in the stretched state. The cover 3 is formed by injection molding of a cover material on the outer surface of a thread wound core of the liquid center 1 and the thread rubber layer 2, or by putting a couple of half-shells (semi-spherical shell), which have been made from the cover material in advance, on the thread wound core to form a spherical cover material which is subjected to compression molding using a mould. As the center bag, thread rubber and cover, there can be used those which are used for this kind of the application. The formulations of the center bag, thread rubber and cover including the formulation of the paste are as follows, but are not limited thereto 1 ○ Formulation of paste Component Amount (Parts by weight) Water 88 Glycerine 12 Bentonite clay 10 to 30 Barium sulphate 50 to 150 The reason why the amount of barium sulfate is within the above range is that the specific gravity is adjusted according to the diameter such that the ball weight becomes no greater than 45.92 g as a specification. It is preferred that the specific gravity of the paste is normally not less than 1.1, particularly in the range of from 1.3 to 2.0. The reason why the amount of bentonite clay is within the above range is that the viscosity is adjusted within a desirable range included in the range of from 15 bo 70 poise. 2 ○ Formulation of center bag: Component Amount (Parts by weight) Natural rubber 100 Filler 30 ± 10 Sulphur + accelerator 4 Also, regarding the center bag, the weight is adjusted by varying the amount of the filler such that the ball weight becomes no greater than 45.92 g as a specification. Preferred examples of the filler include calcium carbonate, barium sulphate, zinc oxide or mixtures of two or more thereof and the like. The production process of the liquid center is not specifically limited, and the liquid center can be produced by the above method which has hitherto been used. The thickness of the center bag is preferably in the range of from 1.5 to 2.0mm, and the hardness of the center bag after vulcanization is preferably in the range of from 40 to 60 (measured by a JIS-A type hardness tester). Regarding the vulcanization conditions of the center (i.e. vulcanization conditions of the rubber composition for center bag), it is preferred that the vulcanization temperature is in the range of from 145 to 165°C and the vulcanization time is in the range of 10 to 40 minutes. However, the vulcanization conditions are not specifically limited, and the time and temperature may be suitably adjusted such that a desirable hardness can be obtained. The size of the liquid center is preferably in the range of from 26 to 32 mm. When the size is smaller than the above range, the amount of spin increases so that it becomes difficult to attain a large flight distance. On the other hand, when the size is larger than the above range, the amount of the thread rubber to be wound becomes small, which results in insufficient hardness of the golf ball. The particularly preferred size of the liquid center is in the range of from 28 to 31.5 mm. 3 ○ Thread rubber: The elastic thread rubber having a thickness of 0.4 to 0.6 mm and a width of 1.3 to 1.8 mm, which is made of natural rubber or isoprene rubber or a blend rubber thereof, was used. It is preferred to increase the diameter of the liquid center in order to decrease the amount of spin of the golf ball, thereby increasing the flight distance thereof. However, when the diameter of the liquid center is increased, the amount of the thread rubber to be wound becomes small and, therefore, it becomes difficult to obtain the requisite hardness. Therefore, it is preferred to obtain the requisite hardness by using a thread rubber having a large stretch ratio. As the thread rubber, there can be used preferably a blend of natural rubber and isoprene rubber, wherein the proportion of isoprene rubber is large, particularly a blend wherein the proportion of natural rubber to isoprene rubber is in the range of from 20:80 to 50:50 in weight. Further, the diameter of the core after winding of the thread rubber is preferably 39.8 ± 0.5 mm. 4 ○ Formulation of cover: Component Amount (Parts by weight) Resin 90 Natural rubber 10 Filler 18 Sulphur + accelerator 2 As the resin, there can be suitably used synthetic transpolyisoprene, gutta-percha, balata, high-styrene resin, 1,2-polybutadiene, transpolybutadiene and the like or mixtures of two or more thereof. From amongst them, the most popular resin is a synthetic transpolyisoprene [TP-301 (trade name), manufactured by Kuraray Co., Ltd.]. Further, the hardness of the cover is preferably in the range of from 70 to 85 (JIS-C type hardness tester). As described above, according to the present invention, there is provided a thread wound golf ball having uniform flight performances which causes little percentage of rejects of the liquid center and inclusion of air into the liquid center, and wherein no separation of the paste arises, by using a paste having a viscosity in the range of 15 to 70 poise at 23° (measured by a B type viscometer) as the paste for liquid center. The following Examples and Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof. Examples 1 to 2 and Comparative Examples 1 to 2 According to the formulation shown in Table 1, a paste was prepared and the paste was injected into a mould and then frozen to prepare a spherical pellet. The pellet was covered with a center bag having a thickness of 1.7 mm to prepare a liquid center having a diameter of 30.0 mm. The viscosity of the paste prepared, the percentage of rejects of the liquid center, the amount of air and the separation state of the paste after standing for 48 hours were examined. The results are shown in Table 1. The respective measurement methods of the viscosity, the percentage of rejects,the amount of air and the separation state of paste are as follows. Viscosity of paste: The viscosity at 23°C of the paste prepared is measured by a B type viscometer. The B type viscometer used is No.1 Rotor of VT-04 manufactured by Rion Co. Percentage of Rejects of center: The weight of the liquid center after vulcanization of the center bag is measured and off-specification centers shall be taken as rejects. The measurement of the percentage of rejects is conducted as to the respective 100 liquid centers of Examples and Comparative Examples. Amount of air: Regarding the liquid center after vulcanization of the center bag, the center bag was broken in water at 23°C and air was collected in a measuring cylinder to measure the amount of air. The amount of air was measured as to the respective 12 liquid centers of Examples and Comparative Examples. The results are shown in the average value thereof. Separation of paste: The liquid center after vulcanization of the center bag was allowed to stand for 48 hours, and then the center bag was broken gently to examine whether the paste was separated into water and filler or not. The separation of the paste was examined as to the respective 12 liquid centers of Examples and Comparative Examples. Regarding the results, the total number of the samples is described in the denominator and the number of the samples wherein separation of the paste arose is described in the numerator In Table 1, "parts" are by weight unless otherwise stated. The formulation of the center bag is as follows, and the vulcanization was conducted by heating at 154°C for 16 minutes under pressure. Formulation of center bag: Component Amount (Parts by weight) Natural rubber 100 Zinc oxide 5 Calcium carbonate 25 Sulphur 2 Accelerator 2 Example No. Comparative Example No. 1 2 1 2 Water 88 88 88 88 Glycerine 12 12 12 12 Bentonite clay 14 17 10 23 Barium Sulphate 64 60 69 54 Viscosity of paste (poise) 30 50 10 80 % of rejects of center 1.5 2.3 4 7 Amount of air (cc) 0.4 0.5 0.6 1.0 Separation of paste 0/12 0/12 8/12 0/12 As shown in Table 1, since the respective pastes of Examples 1 and 2 have a proper viscosity, the percentage of rejects of the center was low and the amount of air was small and, further, no separation of the paste arose. To the contrary, since the viscosity of the paste of Comparative Example 1 is too low, separation of the paste did arise and the percentage of rejects became high. Further, since the viscosity of the paste of Comparative Example 2 is too high, the percentage of rejects of the center was high and the amount of air was large. Then, a thread rubber was wound around the liquid centers of Examples 1 and 2 and that of Comparative Example 1 to form a thread rubber layer, and then a cover was covered on the thread rubber layer to prepare a thread wound golf ball having an average outer diameter of 42.7 mm, respectively. Further, dimples were provided on the outer surface of these thread wound golf balls on cover molding. The number and the total volume of dimples were 400 and 315 mm 3 , respectively. The amount of spin, the flight distance and the deviation in right and left directions of the resulting thread wound golf ball were measured. The results are shown in Table 2. The measurement methods of the amount of spin, the flight distance and the deviation in right and left directions of the resulting thread wound golf ball are as follows. Thread rubber: An elastic thread rubber having a thickness of 0.5 mm and a width of 1.5 mm, which is made of a blend rubber of natural rubber and isoprene rubber (blend ratio = 30:70) was used. Formulation of cover: Component Amount (Parts by weight) Synthetic transpolyisoprene 80 High-styrene resin 10 Natural rubber 10 Zinc oxide 3 Titanium dioxide 15 Sulphur 1.5 Accelerator 0.5 Amount of spin: A golf ball was hit with a metal head club at a head speed of about 45 m/second using a Swing robot manufactured by True Temper Co, and a photograph of the golf ball was taken to determine the amount of spin. The amount of spin was measured as to the respective 8 golf balls of Examples and Comparative Examples. The results are shown by the average value thereof. Flight distance: A golf ball was hit with a metal head driver at a head speed of about 45 m/second using a Swing robot manufactured by True Temper Co, and the distance up to the point where the golf ball was dropped, (carry), was measured. The flight distance was measured as to the respective 8 golf balls of Examples and Comparative Examples. The results are shown by the average value thereof. Deviation in right and left directions: A golf ball was hit with a metal head driver at a head speed of about 45 m/second using a Swing robot manufactured by True Temper Co, and the distance of deviation from the center line in right and left directions of the golf ball was measured. The deviation in right and left directions was measured as to the respective 8 golf balls of Examples and Comparative Examples. The results are shown by the maximum value and the average value thereof (yard). Example No. Comparative Example No. 1 2 1 Amount of spin (rpm) Average value 3230 3280 3220 Flight distance (yard) Average value 228 229 225 Deviation in right and left directions (yard) Maximum value 8 7 12 Average value 4 5 7 As is apparent from the results shown in Table 2, the golf balls of Examples 1 and 2 exhibited large flying distance and small deviation in right and left directions in comparison with Comparative Example 1. The golf ball of Comparative Example 1 exhibited large scattering of the flight distance. As a result, the average value of the flight distance became small and the deviation in right and left directions was large. It is considered that this is because the viscosity of the paste of Comparative Example 1 is too small and, therefore, there was separation in the paste, thereby influencing the flight performances. To the contrary, regarding the golf balls the Examples 1 and 2, the viscosity of the paste is proper and no separation of the paste arose in the liquid center so that scattering of the flight distance is small. As a result, the average value of the flight distance became large and the deviation in right and left directions became small.

Claims: A thread wound golf ball comprising a liquid center (1), a thread rubber layer (2) on the liquid center (1) to form a thread wound core and a cover (3) covering the thread wound core, wherein the liquid center (1) comprises a paste (1a) and a center bag (1b) covering the paste (1a), characterised in that the paste (1a) has a viscosity at 23°C (measured by a B type viscometer) in the range of from 15 to 70 poise. A thread wound golf ball as claimed in claim 1 wherein the paste (1a) comprises a freezing-point depressant, a filler for adjusting specific gravity and a viscosity modifier. A thread wound golf ball as claimed in claim 1 wherein the paste (1a) has a viscosity at 23°C in the range of from 25 to 60 poise. A thread wound golf ball as claimed in claim 2 or claim 3 wherein the freezing-point depressant is glycerin. A thread wound golf ball as claimed in any one of claims 2 to 4 wherein the filler adjusting specific gravity is selected from barium sulphate, sodium sulphate or a mixture thereof. A thread wound golf ball as claimed in any one of claims 2 to 5 wherein the viscosity modifier is selected from bentonite clay, natural hectorite or a mixture thereof. A method of making a thread wound golf ball as claimed in any one of claims 1 to 6, comprising the steps of: (i) forming a liquid center (1) by covering a paste (1a) with a center bag (1b); (ii) forming a thread wound core by winding a thread rubber (2) around the liquid center (1) of step (i) (iii) covering the thread wound core of step (ii) with a cover (3), characterised in that the paste (1a) has a viscosity at 23°C (measured by a B type viscometer) in the range of from 15 to 70 poise.

Inventors: EBISUNO TADAHIRO, HIRAU TSUTOMU, MORIYAMA KEIJI, SUGIMOTO KAZUSHIGE, EBISUNO, TADAHIRO, HIRAU, TSUTOMU, MORIYAMA, KEIJI, SUGIMOTO, KAZUSHIGE
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES, LTD.
Application year: 1994
Main classification: A63B37
IPC codes: A63B37